Human Immunodeficiency Virus (HIV) remains a pressing public health challenge worldwide. Targeting limited HIV prevention and treatment resources effectively with network-based algorithms is a long-standing problem with a rich history at AAMAS and other computational venues. However, optimizing targeted biomedical interventions for minimizing spread of HIV often requires observability of the contact network of HIV transmission risk behavior among individuals, which is prohibitively expensive to collect at scale. Peer referral networks, i.e., networks collected through a recursive peer referral process, are an alternative network class often used to study HIV in hidden populations that can be collected digitally at scale, but they deviate from the underlying contact network: referrals can follow non-risk social ties and participants accept only one referral, hiding contacts among already-recruited individuals. In this work, we first formalize the novel problem of planning network-based interventions to minimize HIV spread in the hidden contact network while observing only the peer referral network. Second, we implement R2C-Agent, a decision making agent for interventions that is designed around our new conditional graph diffusion model called Referral-to-Contact Diffusion (R2C-Diff). R2C-Diff positions the peer referral process as incremental corruption of the underlying contact network and learns to reverse it. R2C-Agent plans its interventions over contact networks sampled from R2C-Diff, so its decisions account for uncertainty about the hidden contact network. Finally, we conduct simulated experiments using real-world HIV network data that demonstrate that R2C-Agent empirically plans more effective interventions than baselines from interdisciplinary literature and highlight key tradeoffs and assumptions driving performance.

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